In recent years, rustproof treatments are performed on the surface of steel sheets for use in such fields as automobiles, home appliances and building materials. In particular, galvanized steel sheets and galvannealed steel sheets that are highly resistant to rust are increasingly used. Further, from the point of view of enhancing the fuel efficiency of automobiles and the safety of automobiles in the event of crash, body materials prefer high-strength steel sheets having higher strength and reduced thickness.
In general, thin steel sheets obtained by the hot rolling or cold rolling of slab are used as the base steel for galvanized steel sheets. The base steel is recrystallized and annealed in an annealing furnace on the CGL (continuous galvanizing line) and is thereafter galvanized. In the case of galvannealed steel sheets, the galvanization is followed by alloying treatment.
The addition of Si and Mn is effective for increasing the strength of steel sheets. However, Si and Mn are oxidized during continuous annealing even in a reductive N2+H2 gas atmosphere which does not cause the oxidation of iron (which reduces iron oxides), forming oxides of Si and Mn on the skin surface of the steel sheets. Such oxides of Si and Mn cause a decrease in the wettability of the base steel sheets with respect to molten zinc during the coating treatment. Consequently, steel sheets containing Si and/or Mn frequently suffer bare spots or, if not bare spots, poor coating adhesion.
Patent Literature 1 discloses a method in which galvanized steel sheets are produced using high-strength steel sheets that contain large amounts of Si and Mn as base steel. In the disclosed method, reducing annealing is performed after an oxide film is formed on the surface of the steel sheets. However, good coating adhesion cannot be obtained stably by the method of Patent Literature 1.
To solve this problem, Patent Literatures 2 to 8 disclose techniques directed to stabilizing the effects by regulating the oxidation rate or the amount of reduction, or by actually measuring the thickness of oxide films formed in the oxidation zone and controlling the oxidation conditions or the reduction conditions based on the measurement results.
In Patent Literature 9, the composition of gases such as O2, H2 and H2O in the atmosphere during the oxidation-reduction steps is specified.
Patent Literature 10 discloses a production method in which a hot-rolled steel sheet is coiled at an increased temperature so as to form Si and Mn oxides in the crystal grain boundaries of the hot-rolled steel sheet.